The Ingenieur is a popular watch, not to mention a true Genta Design. The 3506 is the most attainable iteration of this case design. Out of the few references from the era, the 3506 was the most affordable, without all the bells and whistles.
The 3506 IWC Ingenieur Graph Paper Dial is the only Ingenieur with a dial as such. The Swiss signature distinguishes the 3505 and 3506. Production of the 3506 has been estimated at around 800 examples. The more alternative, neo-vintage set of Ingenieurs that came after the Jumbo 1832 case is increasingly seen as a huge value.
With the larger Ingenieur models not gaining the popularity IWC thought they would have, they slimmed down the case. Thanks to the integrated bracelet, even with an 8.5mm thinness, the watch still wears larger.